Compounds exhibiting intense light absorption in a range of 500 to 700 nm, particularly compounds with wavelength of absorption maximum (λmax) in 550 to 620 nm, are used as optical elements in optical recording layers of optical recording media such as DVD-R and in optical filters of image display equipment such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescence displays (ELD), cathode-ray tube displays (CRT), fluorescent display tubes, and field emission displays.
As the optical element, there have been studied a number of indole ring-containing cyanine compounds with highly sensitivity. A number of such cyanine compounds have been reported particularly as recording elements of optical recording media including DVD-R as a representative, owing to a merit of capability to follow speed-up of recording. For example, they are reported in Patent Documents 1 to 5 below. Patent Document 6 also describes a cyanine compound, in which a benzyl group is introduced at 3-position in the indole ring.
However, conventional cyanine compounds have a problem in thermal decomposition properties. Compounds with low decomposition temperature would be suitable as recording elements of optical recording media for high-speed recording, but the above cyanine compounds do not have sufficient properties in this aspect.
Patent Document 1: Japanese Patent Laid-Open Publication No. H10-278426
Patent Document 2: Japanese Patent Laid-Open Publication No. H11-227331
Patent Document 3: Japanese Patent Laid-Open Publication No. H11-277904
Patent Document 4: Japanese Patent Laid-Open Publication No. 2001-506933
Patent Document 5: Japanese Patent Laid-Open Publication No. 2002-52829
Patent Document 6: Japanese Patent Laid-Open Publication No. 2003-231359